Field of the Invention
The present invention relates to light intensity sensors of the type comprising a variable-voltage opto-electronic generator adapted, as a function of the variable light intensity picked up by the sensor, to deliver a variable voltage to the terminals of a load resistor.
Conventional sensors of this type display some advantageous features in connection with the use of a variable-voltage opto-electronic sensor. However, such conventional sensors are disadvantageous for a number of reasons. First, the voltage available across their output terminals is usually affected by line resistances and also by radioelectric interferences from the main power lines. Second, the output terminals are usually polarized so that inadvertently connecting the conventional circuits to a power supply of reversed polarity may damage the sensor, and in particularly, the opto-electronic generator used in the sensor. Third, this variable voltage is extremely low, in the order of 0.5 V, for example, so that it is not possible to include in such a sensor a diode bridge to eliminate the polarization of its terminals. In fact, a diode bridge would by itself cause a voltage drop of about 1.2 V, which is inconsistent with the 0.5 V voltage available across the output terminals of the conventional sensor. Furthermore, the very low voltage obtained across the sensor output terminals can only be measured by using very sensitive and, therefore, expensive instruments. Fourth, when the light intensity received by a conventional sensor is zero, the sensor does not deliver any quiescent or "rest" current and it is not possible, unless additional apparatus are used, to know at any time if the electrical connection between the sensor and the measuring instrument is broken or not.